Impala Platinum's attempt to mop up the minority interests in Zimbabwe Platinum has suffered a major blow with Zimbabwe-listed Barbican Holdings apparently preparing a rival $5 a share offer.

The offer compares with Impala's $4.08 per share or $195 million offer aimed at securing Zimplats' stake in the Ngezi open-cut mine and the Selous Metallurgical Complex in Zimbabwe and access to more than 165 million ounces of platinum.